Memory is one type of integrated circuitry, and is used in computer systems for storing data. Memory may be fabricated in one or more arrays of individual memory cells. Memory cells may be written to, or read from, using digit lines (which may also be referred to as bitlines, data lines, sense lines, or data/sense lines) and access lines (which may also be referred to as wordlines). The digit lines may conductively interconnect memory cells along columns of the array, and the access lines may conductively interconnect memory cells along rows of the array. Each memory cell may be uniquely addressed through the combination of a digit line and an access line.
Memory cells may be volatile or nonvolatile. Nonvolatile memory cells can store data for extended periods of time including when the computer is turned off. Volatile memory dissipates and therefore is rapidly refreshed/rewritten, in many instances multiple times per second. Regardless, memory cells are configured to retain or store memory in at least two different selectable states. In a binary system, the states are considered as either a “0” or a “1”. In other systems, at least some individual memory cells may be configured to store more than two levels or states of information.
A capacitor is one type of electronic component that may be used in a memory cell. A capacitor has two electrical conductors separated by electrically insulating material. Energy as an electric field may be electrostatically stored within such material, and the electrical state of the material may be utilized to represent a memory state. Accordingly, the insulative material of a capacitor may correspond to a programmable material of a memory cell. The insulative materials conventionally utilized in capacitors may dissipate charge quickly, and accordingly tend to be utilized in volatile memory.
One type of capacitor is a ferroelectric capacitor which has ferroelectric material as at least part of the insulating material. Ferroelectric materials are characterized by having two stable polarized states. The polarization state of the ferroelectric material can be changed by application of suitable programming voltages, and remains after removal of the programming voltage (at least for a time). Each polarization state has a different charge-stored capacitance from the other, and which ideally can be used to write (i.e., store) and read a memory state without reversing the polarization state until such is desired to be reversed. Less desirable, in some memory having ferroelectric capacitors the act of reading the memory state can reverse the polarization. Accordingly, upon determining the polarization state, a rewrite of the memory cell is conducted to put the memory cell into the pre-read state immediately after its determination. Regardless, a memory cell incorporating a ferroelectric capacitor ideally is nonvolatile due to the bistable characteristics of the ferroelectric material that forms a part of the capacitor. One type of memory cell has a select device electrically coupled in series with a ferroelectric capacitor.